Blood Adv. 2026 Mar 04. pii: bloodadvances.2025018716. [Epub ahead of print]
Vaso-occlusion is a signature pathology of sickle cell disease (SCD). However, the lack of in vivo methods to observe individual blood cell dynamics in humans limits our understanding of occlusion formation mechanisms. We present a novel in vivo, non-invasive, label-free, and high-resolution imaging technique to study blood flow and sickled cell behavior in affected individuals. We used oblique back-illumination microscopy (OBM) to capture videos of 91.0 ± 42.3 sublingual capillaries in each of ten subjects with SCD before and after red cell transfusions and compared measurements to ten unaffected controls. With direct observation of blood cell activity, we identified microvascular occlusions initiated by red blood cells (RBCs) that adhered to the endothelium and caused mechanical vessel obstruction. Often, the RBCs were sickled. Then, in each observed vessel, we classified blood flow as fast, slow, or no flow, and counted adhered RBCs. Compared to controls, SCD subjects before transfusion had fewer fast-flowing vessels (48.7% vs. 77.7%, p=5.8x10-4), more no flow vessels (16.1% vs. 2.4%, p=0.0010), and more adhered RBCs (1.37 vs. 0.01 cells per vessel, p=0.0025). From before to after transfusion, SCD subjects' microvasculature had increased fast-flowing (48.7% vs. 65.8%, p=0.0098) and decreased no flow vessels (16.1% vs. 6.0%, p=0.0039); adhered RBCs decreased (1.37 vs. 0.71 cells per vessel, p=0.043). These hemorheological indices captured transfusion-induced changes to vascular dynamics and events leading to microvascular dysfunction and occlusion in SCD. Our findings demonstrate the potential of OBM to study vaso-occlusion pathobiology, accelerate therapeutic evaluation, and personalize treatment strategies in people with SCD.